A bitmapped image as a mask as seen in the Layers panel. The mask thumbnail
shows that the image is automatically inverted.

Stop Destroying Innocent Pixels One of the most important reasons to mask a scanned image, rather than
cutting or cropping, is that you preserve the original pixels in the image.
The benefit of the mask is that you still have the original image to go
back to - not so if you delete.

There's one last option for using a bitmap image as a mask - use the alpha
channel transparency as the mask shape. This is very useful when you have a
bitmap image with a unique shape on a transparent background. You can use the
image's shape to mask other objects.

To mask to an image's alpha channel:

Position the image you want to use as the grayscale mask over the objects
that are to be masked (Figure 17).